“Extraordinary measures are projected to be exhausted in the middle of October,” Lew said in the letter today to House Speaker John Boehner and other lawmakers.

“At that point, the United States will have reached the limit of its borrowing authority, and Treasury would be left to fund the government with only the cash we have on hand on any given day,” he said. The cash balance at that time is forecast to be about $50 billion, “insufficient to cover net expenditures for an extended period of time,” according to Lew.

The Treasury Department had earlier said it probably will be able to finance government operations by using special accounting measures until after Congress returns Sept. 9 from its recess. Lew said Aug. 22 a failure by Congress to raise the debt limit would “have disastrous effects for our nation” and could put at risk payments to Social Security recipients and veterans.

“Congress should act as soon as possible to protect America’s good credit by extending normal borrowing authority well before any risk of default becomes imminent,” Lew said in the letter.